The study drug, VX15/2503, is a monoclonal antibody that binds to the semaphorin 4D (SEMA4D; CD100) antigen. Semaphorins have been shown to play an important role in certain physiological processes such as vascular growth, tumor progression and immune cell regulation. Experimental evidence suggests that SEMA4D has two mechanisms of action that result in angiogenesis and tumor proliferation and invasion. Antibody neutralization of SEMA4D thus may represent a new therapeutic strategy for cancer treatment.